1. Field of the Invention
The present invention relates to a semiconductor laser.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication Nos. 2006-245346 and 2008-066318 disclose wavelength-tunable semiconductor lasers used in optical communication systems. The semiconductor laser disclosed in Japanese Unexamined Patent Application Publication No. 2006-245346 includes a plurality of ring resonators having optical path lengths different from each other, wherein the plurality of ring resonators are optically connected to each other. The semiconductor laser also includes a light input/output unit configured to input and output a laser beam to and from the plurality of ring resonators. These ring resonators have free spectral ranges (FSRs) different from each other. Laser oscillation occurs at a wavelength at which the transmission peak wavelengths of the respective ring resonators overlap each other. Furthermore, the lasing wavelength can be changed by changing the transmission peak wavelengths of the ring resonators by injecting a current into the ring resonators. In this case, a so-called vernier effect is used for changing the lasing wavelength. Furthermore, the lasing wavelength is monitored and it is controlled on the basis of the monitoring results.
In the semiconductor laser disclosed in Japanese Unexamined Patent Application Publication No. 2008-066318, ring resonators are connected at the front and the back of a gain region. The FSRs of these ring resonators are different from each other, and laser oscillation occurs at a wavelength at which the transmission peak wavelengths of the ring resonators overlap each other. The transmission peak wavelengths of the ring resonators can be changed by injecting a current into the ring resonators. That is, as in the semiconductor laser disclosed in Japanese Unexamined Patent Application Publication No. 2006-245346, the lasing wavelength can be changed by injecting a current into the ring resonators using the vernier effect.
Kimoto et al. “Highly Reliable 40-mW 25-GHz×20-ch Thermally Tunable DFB Laser Module, Integrated with Wavelength Monitor”, Furukawa Review, No. 24, 2003, pp. 1-5 discloses a laser module provided with a wavelength-tunable DFB laser and a wavelength control element (wavelength locker). The wavelength locker in this laser module includes a prism, two photodiodes, and an etalon filter. In this laser module, the prism branches backlight of the DFB laser into two light beams. The etalon filter receives one of the light beams and selectively passes light of a certain wavelength. One of the photodiodes directly receives the light beams from the DFB laser through the prism. The other of the photodiodes receives the light passing through the etalon filter.